Comparative studies of chicken gizzard and rabbit skeletal tropomyosin

Interaction of tropomyosin and troponin T: a proton nuclear magnetic resonance study

Proton nuclear magnetic resonance (1H NMR) has been used to study the nature of the interaction between tropomyosin (TM) and troponin T (Tn-T). Resonances corresponding to the histidine residues in fragments of both TM and Tn-T can be resolved and assigned in the 1H NMR spectrum. Changes in the pH titration profiles of these resonances when the various fragments are mixed provide probes of the interaction sites between the proteins. Fragment T1 (residues 1-158) of Tn-T appears to interact weakly but specifically with fragments of TM in which the NH2-terminus (residues 1-10) is intact. While fragment CB2 (residues 71-151) of Tn-T interacts weakly (dissociation constant of 0.1-0.2 mM) with NH2-terminal fragments of TM, this appears to be nonspecific since the absence of residues 1-10 and 128-189 of TM does not affect the observed perturbations of the titration profiles of His-79 of CB2. Although a strong interaction between T1 and the COOH-terminal Cy2 fragment (residues 190-284) of TM has been previously demonstrated, no perturbation of His-276 of Cy2 or of His-7, -23, -29, or -36 of T1 was observed in a mixture of T1/Cy2. The pKa of His-276 was also not affected in a mixture of Cy1/Cy2 (where Cy1 is residues 1-189 of TM) but was significantly decreased in the ternary complex T1/Cy1/Cy2. The importance of residues 1-70 of Tn-T in its binding to TM is illustrated by the specificity it confers on the T1/Cy1 interaction and by the absence of His-276 perturbation in the mixture CB2/Cy1/Cy2.

Dual effect of filamin on actomyosin ATPase activity

Filamin binds to F-actin and influences the myosin-actin interaction. At relatively low concentrations, filamin activates actomyosin Mg2+-ATPase, whereas higher concentrations of filamin exert an inhibitory effect. Activation of ATPase activity occurs under conditions where a loose meshwork of actin filaments is present and inhibition is associated with the appearance of closely apposed bundles of actin filaments. Maximum activation (about fourfold) of actomyosin ATPase activity by filamin occurs between 30 and 65 mM KCl, at pH 6.5, and at temperatures not less than 30 degrees C. ATPase activation requires higher concentrations of filamin in the presence than in the absence of tropomyosin. Filamin does not activate Mg2+-ATPase activity of acto-subfragment-1 and has only a slight effect on the Mg2+-ATPase of acto-heavy meromyosin, but it inhibits the activity of both these systems under conditions similar to those that inhibit actomyosin ATPase activity.